Advise from a fellow NJ Surf Dad: Do not put your boy on an over sized and over foamed board. He will learn bad techniques that will last a lifetime. Better to keep him riding that beater board quite frankly. If you think he is ready for a proper hard board I suggest you build or get him a properly proportioned short board that is no more than a couple inches longer than he is tall.
I would take a close look at the Channel Islands Grom Series boards and build something similar. I will warn you that making a grom sized board is far more difficult than building an adult size board. Take that from someone who's built a lot of grom size boards over the past 10 years. https://www.cisurfboards.com/grom-series/
Just my humble two cents as the father of 3 children one of whom I made the mistake of starting on what amounted to a mini-mal.
Pro-tek safety fins. always, for any kid board. Nothing will put them off surfing like a fin cut.
My third design didn't land far off from the channel island design. Just a hair shorter, thicker and wider, at 19L vice their 15L 4'8". He will be under 45 lbs next summer, so if you think I could stand to go lighter still, it does look pretty thick.
I think I would approach this in one of two ways.
A. Study kneeboard shapes and you just may find something that could be adapted to your son's size and weight.
B. Find a fun board or egg shape in the 7'" to 7'-6" range and size it down to the 5' range.
In either case make the board something that can catch waves easily. Wave count is more important at your son's stage of ability than being able to rip like Slater.
Kids adapt much more quickly than us older folks. Give the kid a board that might be a challenging shape for you to ride and guess what.....a kid will adapt and learn to ride it quickly. Put the kid on a oversized canoe for the sake of being able to boost wave count and the kid can get away with any sort of bad technique when popping up or riding because the canoe lets them, get away with it. Now is when technique is learned and becomes ingrained for life. Now is also when the child can adapt and learn quickly to pop up and ride a shortboard properly.
Right, but what if your kid is so small that every board you have or could be purchased is an oversized canoe? In that case, what would the perfect learner board be if you were shaping one? What shape and volume? That's my dilemma. Right now, based on input here, I have a 4'6, 16", 2", at 17L in a slightly narrower and shorter McCoy nugget shape.
It's all about the ride. The "oversized canoe" will give him stability and confidence. Let's not forget he is a 5 year old. I'd save the rip and slash for later.
The rip and slash will come later. The thing is if the kid becomes a knee dragger at this age he will probably always be a knee dragger. Just based on my experience the child will best learn to pop up properly if the board requires it. Beware the board that lets the grom crawl to his feet and get away with it.
This young lad that my son surfs with rides a 4'2
Here is a shot when he was younger. Not sure what the dims were on this board. At that stage his dad was still pushing him into waves. He is the tiniest kid I've seen who could really surf. He's a seasoned vet now at 8.